When I click it, it disappears,
Why would that be? Is it set up as a button to perform a script that changes the value of the field?
It's pretty difficult to follow all the details of your description here. As an example, there are times when you write something like "This Portal..." and it's tricky to figure out which of two portals you are referring to.
Understandable. In all cases but the first, I'm referring to the portal displaying Table1_Table2_selected_Table3 records. There is no script associated with the disappearing field, only a conditional visibility that hides it when "Table1_Table2_selected_Table3::condition ≠ 1". Since the portal row (T3) that this popover is inside has condition =1, it should be visible, which it is, until it's in focus.
The field is part of Table1_Table2_selected, not table 3. I think because of that, it's looking at the condition in the first record of Table1_Table2_selected_Table3 (which has condition=2, let's say) instead of the current one. Should it be? Should the current portal row be in focus because I'm in its popover, or should it not be because I'm in the Table2 field?
Either way, it works correctly for some other fields and it always worked in FM13.